<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            font-size: 100px;
        }

        @media screen and (max-width: 800px) {
            html {
                font-size: 80px;
            }
        }

        @media screen and (max-width:400px) {
            html {
                font-size: 50px;
            }
        }

        .font-size-test {
            font-size: 1rem;
        }
    </style>
</head>

<body>
    <div class="font-size-test">Hello World!</div>
</body>

</html>